During embryonic development, genes are expressed under a strict spatial and temporal order in cells and tissues. This regulation is governed by regulatory regions in the genome, usually identified as enhancers (Kondoh, 2008). The identification and mapping of a set of enhancers allow clarification of essential regulatory elements involved in the enhancer action and their interacting protein factors. Enhancer analysis also determines upstream signaling cascades that regulate interacting protein factors. If the regulatory regions do not function properly, spatio-temporal order of the gene expression will be disrupted, and this may cause abnormal development and dis eases (Kleinjan & van Heyningen, 2005; Sabherwal et al., 2007). Thus, identifica tion of the regulatory regions provides an important entry point to clarify regulatory mechanisms underlying embryonic development.

In the investigation of regulatory regions associated with a gene, various animal systems and tissue-cultured cells are widely employed, each having unique advan tages and drawbacks. We find that electroporation of chicken embryos is most advantageous for systematic survey of development-associated regulatory regions (Uchikawa et al., 2003, 2004; Uchikawa, 2008). The chicken embryo has advan tages in developmental studies (Stern, 2004), for its amenability to live observation and tissue manipulation, and availability of simple culture system (New's culture), and application of in ovo electroporation (Muramatsu et al., 1996; Funahashi et al., 1999) certainly enhances its merit.